1. Field of the Invention
The present invention relates to a suspension substrate used for a hard disc drive (HDD), and, particularly, to a suspension substrate allowing the whole thickness to be small.
2. Description of the Related Art
A recent distribution of internet and the like have created demands for increases in the amount of information processing and in information processing speed of a personal computer. Along with this trend, it has come to be necessary that hard disc drives (HOD) incorporated into a personal computer be increased in capacity and in rate of information throughput. Then, the part called “magnetic head suspension” which is used in this HOD for supporting the magnetic head is also shifted from a conventional type obtained by connecting signal lines such as gold wires to a wire-integrated type (flexure) called “wireless suspension” which is obtained by forming signal lines such as copper wires directly on a stainless spring. Also, it has come to be necessary to design the suspension substrate as a distributed constant circuit along with increase in rate of information throughput.
The wirings formed on such a suspension substrate are usually formed in pairs on the same surface of the insulation layer as shown in Japanese Patent Application Laid-Open No. 2005-11387. Usually, a read wiring made of a pair of wirings and a write wiring made of a pair of wirings are formed on the same surface of the insulation layer. In such a pair of wirings, electric signals are transmitted by differential transmission, and the differential impedance which is the characteristic impedance of a differential transmission line as the distribution constant circuit exists between the pair of wirings. It is demanded of this differential impedance to be lowered from the viewpoint of impedance matching along with the development of a magnetic head and preamplifier having a lower impedance.
Here, as mentioned above, in the case of a suspension substrate having a pair of wirings on the same surface of the insulation layer, it is necessary to make the wiring have a very large width (for example, hundreds of micrometers) or to make the insulation layer have a lower thickness (for example, 5 μm or less) to reduce the differential impedance. However, the increase in the width of the wiring is undesirable from the viewpoint of density growth and the decrease in the thickness of the insulation layer poses the problem concerning an increase in the occurrence of pinholes.